This invention concerns a process for exothermic heterogeneous synthesis in which the synthesis gas runs through a series of catalytic beds stacked but separate from one another contained within the same reaction space, where the gas leaving one bed runs through the next catalytic bed.
The invention also concerns reactors for this process, consisting of a pressure-resistant outer shell, of baskets for catalytic beds all inside the same shell, of a possible cartridge and of a heat regenerator.
In the production of ammonia the amount of heat which develops from the synthesis reaction of N2+3H2 is remarkable, such heat being generally recovered for the final purpose of producing steam used in the production cycle in order to reduce energy consumption.
The latest technologies aim at the maximum recovery of this synthesis heat at the highest degree of heat possible; synthesis units and their principal piece of equipment, the reactor, are therefore suitably designed for this purpose. In newly built plants, the reactors have several catalytic beds with intermediate cooling of the gas by means of indirect exchange through heat exchangers; in addition, part of the reaction heat is removed with an external cooling agent such as for example the feed water to the boiler or by generating steam, before the last reaction stage, in order to be able to generate at the maximum temperature possible (heat recovery at the highest degree of heat) without limiting the maximum reaction yield achievable.
Maximum temperature together with maximum yield are in effect two opposing requirements, as is widely demonstrated by the diagrams which show in abscissa the concentration of ammonia and in ordinate the temperature of the gas.
The most important designers of synthesis reactors have generally favored reactors with several catalytic beds in at least two distinct apparatus units in series, in order to satisfy the above-mentioned need for the optimal exchange of reaction heat (at the highest degree of heat possible) without limiting the maximum yield achievable (Fertilizer Focus, October, 1987).
In the case of two separate items or units of equipment, the first of these contains generally two catalytic beds with intermediate indirect cooling by means of an internal exchanger, while the second item or unit of equipment generally contains a single catalytic bed.
Between the two items of equipment heat exchange is achieved by introducing a boiler to produce stem. This is the case for the Topsoe Series 250 (Series 200 1 Series 50) reactor and for the Uhde reactor, both with radial flow of the gas in the catalytic beds (Fertilizer Focus, October, 1987, pages 36 and 39).
There are also reactors designed in three separate parts, each containing a catalytic bed with axial gas flow according to the design by C. F. Braun (Nitrogen Conference, Amsterdam 1986). In this case a boiler for the production of steam is introduced between the second and the third reaction unit (Nitrogen Conference, Amsterdam 1986, Mr. K. C. Wilson, Mr. B. J. Grotz, and Mr. J. Richez of CdF Chimie).
According to a recent patent by C. F. Braun (UK Patent Application 2132501A), the gas/gas exchanger between catalytic beds, which is normally conveniently situated inside the reactors with at least two beds within a single unit, is situated outside the reaction unit directly connected at the bottom of the shell containing a single catalytic bed.
To minimize the problem of tubes at a high temperature, the tube connecting the above-mentioned horizontal exchanger with the shell containing the catalytic bed is cooled by the fresh gas fed to the reactor.
After having pre-heated the fresh feed gas, the gas leaving the catalytic bed leaves the exchanger and feeds the unit containing the second catalytic bed (C. F. Braun reactor with several reaction units, as shown in FIG. 5 of the Wilson, Gritz, Richez report in the above-mentioned reference and on page 48 of Fertilizer Focus, October, 1987).
The problem solved in the above-mentioned C. F. Braun Patent, i.e. the avoidance of high temperature gas coming into contact with the tubes connecting shell and exchanger, does not exist in reactors with several catalytic beds in a single unit since, as mentioned above, the gas/gas exchanger is located directly inside the reactor itself.
C. F. Braun solves the problem of optimal recovery of heat is solved in a complex way by introducing a boiler connected by a complex tube arrangement to the reactor itself (see FIG. 5 in the C. F. Braun Nitrogen '86 presentation and Fertilizer Focus October, 1987, page 48).
All the above designs, although they solve the thermodynamic problem, are very complex and therefore very expensive.
Ammonia synthesis reactors operate in fact at high pressure, generally not below 80 bar, and more often between 130 and 250 bar, and at a temperature (400.degree.-500.degree. C.). The connecting tubes of the various items of equipment necessary according to the schemes described above (as schematically shown in the above-mentioned references) operate under critical conditions (high gas temperature between the various reaction beds) and must therefore be made of special material and with long runs to minimize mechanical stress produced by thermal expansion. The situation is particularly complex in reactors according to C. F. Braun, in spite of the measures taken according to the C. F. Braun UK application No. 2132501A.
The assignee of the present application, continuing its research in this field, has found a process and reactor with several catalytic beds free from the disadvantages described above, which can be constructed in a single apparatus, permitting the easy exchange of heat between catalytic beds, and in particular before the last catalytic bed, in order to achieve maximum recovery of reaction heat at the highest possible degree of heat, said heat being recovered for example to pre-heat the boiler water for the direct production of steam.
In an advantageous embodiment, the hot gas reacted in the last catalytic bed but one is transferred, through a duct usually placed axially in a vertical reactor, directly to the heat recovery system (pre-heater or boiler), to be then returned directly to the reactor by means of a duct either external or internal to the above-mentioned transfer duct, creating an airspace for the gas returning to the reactor, which gas then feeds directly the last catalytic bed with an axial-radial or radial flow either centripetal or centrifugal. Said gas, after being reacted in the last catalytic bed, is then transferred again to the central or external part of the reactor, leaving then from the bottom of the reactor.